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Regulatory Matters - Utility Rates (Details) $ in Millions |  |  | 3 Months Ended
--- | --- | --- | ---
Jan. 01, 2013 | Jun. 01, 2011 | Mar. 31, 2016 USD ($) Customers Years MW | Mar. 31, 2015 USD ($)
Minnesota Power [Member] | Bison Wind Energy Center [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Generating Capacity (MW) | MW |  |  | 497 |
PSCW [Member] | 2012 Wisconsin Rate Case [Member] | SWL&P [Member] | Retail [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Approved Return on Common Equity | 10.90% |  |  |
Electric Rates [Member] | MPUC [Member] | 2010 Minnesota Rate Case [Member] | Minnesota Power [Member] | Retail [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Approved Return on Common Equity |  | 10.38% |  |
Approved Equity Ratio |  | 54.29% |  |
Electric Rates [Member] | MPUC [Member] | Minnesota Cost Recovery Riders [Member] | Minnesota Power [Member] | Retail [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Revenue from Cost Recovery Riders | $ |  |  | $ 25.4 | $ 20.7
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Minnesota Power [Member] | Retail [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
North Dakota Investment Tax Credits | $ |  |  | 8.0 |
Electric Rates [Member] | MPUC [Member] | Annual Automatic Adjustment of Charges [Member] | Minnesota Power [Member] | Retail [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Retail Fuel Cost Recovery Collected but Subject to Refund | $ |  |  | $ 350.0 |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Number of Customers | Customers |  |  | 16 |
Length of Notice Required to Terminate Contract (Years) | Years |  |  | 3 |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Expire December 2024) [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Number of Customers | Customers |  |  | 14 |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Expire December 2024) [Member] | Maximum [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Change in Annual Capacity Charge Per Contract - Percentage |  |  | 2.00% |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Expire December 2024) [Member] | Minimum [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Change in Annual Capacity Charge Per Contract - Percentage |  |  | (1.00%) |
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contract (Termination Effective June 2019) [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Number of Customers | Customers |  |  | 1 |
Average Monthly Demand (MW) | MW |  |  | 29 |

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